1. Field of the Invention
The invention is in the field of integrated optic devices and photolithographic techniques for their fabrication.
2. Brief Description of the Prior Art
A great deal of effort has recently gone into the development of signal processing devices for optical communication systems. Much of this effort has gone into the development of optical devices which can be fabricated by photolithographic techniques on planar substrates with a view to the integration of large arrays of devices on large area substrates. The substrates are transparent to the optical signals being employed. The optical signals are transmitted along strip waveguides produced by the deposition of material on the substrate or the diffusion of material into the substrate in order to locally modify its optical properties.
One basic device in such an integrated optic system is the optical directional coupler. One such coupler is produced by the formation of two strip waveguides which are in close proximity with one another, over a defined length (Marcatili, Bell System Technical Journal, 48 (1969) 2071). In such a coupler some or all of the light introduced into one strip guide is transferred to the other guide. These directional couplers have been used to fabricate electrically actuated switches and modulators by, for example, the fabrication of the coupler and a set of electrodes on an electrooptic material. The coupled length is chosen to produce full power coupling from one strip to the other in the absence of a voltage bias across the electrodes. The electrodes are so situated that a voltage bias produces a difference in the optical propagation constant in one guide relative to that in the other guide. This difference of propagation constant (often referred to as asynchronism) results in the reduction of light coupling from one guide to the other. The magnitude of the applied bias is chosen so as to reduce the coupling to as nearly zero as possible. In this way the application of the switching bias switches the light output from the coupled guide to the originating guide (Zernicke et al., Journal of the Optical Society of America, 62 (1972) 1346, Papuchon et al., Applied Physics Letters, 27 (1975) 289). A modification of this type of switch is disclosed in copending application Ser. No. 641,649. In this switch the biasing electrodes are separated into two longitudinal portions which are oppositely biased. This produces a switch which is, among other things, less critical of manufacture.
Another type of integrated optical switch, taught in the art, incorporates two 3 dB couplers (i.e., one-half of the incident power emerges from each of the two output ports). The two 3 dB couplers are linked by two strip guides with a phase modulator being incorporated in one or both of these intermediate guides. The introduction, via an electrical bias voltage, of a .pi. relative phase shift, switches the light output from one port to another of the composite device (Kaminow et al., Applied Physics Letters, 27 (1975) 555; Zernicke, Digest of Technical Papers of the Topical Meeting on Integrated Optics, New Orleans (1974) WA5). The utility of such a switch for a communication system depends, in part, upon the ability to limit the crosstalk between communication channels to an acceptable level by switching all of the power from one port to another. In this type of switch, the ability to limit crosstalk depends, in part, upon the accuracy of the 3 dB split of each coupler. The line widths and spacings characteristic of such devices (e.g., 1-5 micrometers) make it difficult to consistently achieve accurate 3 dB splits merely by close control of manufacturing tolerances. Zernicke (referred to above) suggests adjusting the length of each coupler after its initial fabrication to produce the desired equality of power split. However, this could be burdensome for large-scale multi-element systems.